The present invention relates to a direct-pull multistage wiredrawing machine.
Multistage wiredrawing machines are known in which the wire to be drawn is successively passed through drawbenches having decreasing sections. In order to avoid tensions to be generated that would bring to the wire breakage, it is necessary to keep the product of the wire section by the wire speed constant. Therefore rollers are generally used over which the wire length located between two drawbenches is wrapped and which are driven in rotation by electric motors operated in such a manner as to produce the traction action necessary for that wire length. Several different solutions have been proposed for performing the function of adjusting the motors' speed. For example it has been proposed to carry out the motors' adjustment (either manually or with the aid of automatic or semiautomatic devices) so as to produce a torque slightly lower than that necessary to start drawing. After that, the last motor of the drawing machine is adjusted for supplying a pulling action enabling the immediately preceding motor to rotate, so that, in turn, the last mentioned motor will produce a pulling action transmitted to the motor immediately preceding it, and so on as far as the whole drawing train is set in motion. Such a drawing machine can be used only for working wires which are sufficiently big to withstand the pulling forces that are generated during transient steps and which produce spreading of "jerks" along the different stages. Also proposed have been wiredrawing machines having sensors disposed along the wire lengths between the rollers in order to detect the wire tension and drive the motors so as to keep said tension to an acceptable value. Usually said sensors consist of feeler pins or takeup rollers on which the wire rests and over which it is wrapped. The sensor movement is converted to electric signals piloting the speed control devices of the electric motors. In addition to the mechanical complexity introduced by the plurality of movable feeler pins or takeup rollers disposed along the wire lengths, the deformations imposed to the wire by such sensors in order to be able to detect tensioning of same submit the wire to undesired stresses.
"Storage" drawing machines have been also proposed in which, in place of the speed control, wire storing means is used for accumulating the wire between the different drawing machines, so that stages are isolated from one another as much as possible and spreading of pulls is avoided, However, wire storing gives rise to unavoidable wire bendings and twistings that, together with the uncontrolled pulling actions that can take place in the storing means itself, cause concealed tearings in the material or even breakage of the wire at the solderings, or microdefects.
In the Italian Patent Application No. MI91A001584 filed in the name of the same applicant a multistage wiredrawing machine is described. Each pulling roller is fitted on a gear meshing with a planetary gear rotating about the former. Kinematically connected to the planetary gear is, in turn, one pulley passed over by a length of a belt driven by a second pulley fitted to a motor. The second pulley is an expanding pulley, that is its diameter varies on varying of the belt tension so that increasing or decreasing in the torque transmitted between the gears causes the pignon to rotate about the gear thereby tensioning or releasing the belt and therefore varying the transmission ratio between the pulleys.
While such an embodiment offers a fair adjustment of the wire tensions, it has a relatively expensive mechanical structure and relatively high intervention thresholds, as the wire tension has to overcome the value necessary to impose the diameter variation in the expanding pulley. In addition, it has a rather important bulkiness. The general object of the present invention is to eliminate the above mentioned drawbacks by providing a multistage wiredrawing machine provided with a system for controlling the wire speed and tension in working, which system is capable of preventing abnormal stresses in the wire while being of simple mechanical structure, quick intervention and very high sensitivity.